The invention relates to a method and an apparatus for the suppression of interference signals in a useful signal, wherein the interference signals can occur as a mixture of narrow interference pulses having a high amplitude and interference hum, and whereby the presence of interference pulse peaks and/or interference hum can be separately recognized by means of interference peak and interference hum recognition installations, and, with said recognition, suppressor installations for suppressing the interference pulse peaks and the interference hum in the useful signal are activated.
Particularly in medical technology, wherein e.g. physiological signals are tapped from a patient, interference voltages, such as base line fluctuations, interference voltage peaks resulting from muscle spasms or muscle tremblings, or resulting from artificially supplied stimulation pulses, and mains (or power supply) frequency interferences are frequently observed in the signal derivation as higher-frequency interference hum. Specifically in the case of derivation of an electrocardiogram, pacemaker pulses of a heart pacemaker can also be superimposed on the EKG-signal, in addition to a power supply hum. This applies, in particular, always to those patients who, in order to support their cardiac activity, wear a heart pacemaker. Pacemaker pulses in the EKG are not in every instance an undesired interference voltage; since they are carriers of the information regarding an effected stimulation of the heart, in special instances, it may be desirable, for the purpose of specific evaluation (or analysis), to reproduce EKG-signals together with heart pacemaker pulses. In the normal case; i.e., in common practical application, such as heart frequency measurement, EKG recording or display, etc., however, the presence of pacemaker pulses has a considerably interfering (or disturbing) effect; through the pacemaker pulses, namely, the information content of the EKG signal is generally altered to such an extent that its evaluation (or analysis), whether it be visually by means of the eye, or automatically by means of a suitable evaluation (or analysis) unit, is rendered difficult or even becomes impossible. In these instances, the peak interference voltage; e.g. the pacemaker pulse, must be suppressed in the useful signal.
In general, such an interference peak suppression presents no difficulties insofar as additional interference components such as e.g. the power supply hum are not to be also simultaneously suppressed. An apparatus which permits a suppression of interference pulse peaks in a useful signal; i.e., specifically also of heart pacemaker pulses in an electrocardiogram, is prior knowledge from the British Pat. No. 1,450,081. The suppression of the interference pulse amplitudes however, occurs therein without the simultaneous suppression of an interference hum component. Correspondingly, e.g. the apparatus of the German-AS 2,558,126 also permits only the suppression of power supply hum in the EKG without the presence and simultaneous suppression of interference peaks, such as pacemaker pulses or the like. An apparatus which renders possible a suppression of interference pulse peaks as well as interference hum in a common circuit arrangement is, indeed, already prior knowledge from the U.S. Pat. No. 3,950,694; however, precisely in the case of such a circuit arrangement, reactions of the blanking out of interference amplitude peaks on the suppression of the power supply hum cannot with certainty be precluded in advance. Thus, the danger exists that, during the period of suppression of an interference peak amplitude, the recognition installation, or also the suppression installation for the interference hum may be affected in an uncontrollable manner due to the interruptions occurring, or also due to the jumps in the signal pattern (or progression, or curve). Conversely, however, a useful signal which has not yet, or only insufficiently, been refined of the interference hum has an unfavorable effect in terms of the recognition of interference pulse peaks and their suppression. In the case of heart pacemaker pulses, there then result e.g. difficulties in the compensation particularly of the pacemaker overshoot, since it is hardly possible to convert an EKG with hum interferences into an e-function curve or to derive from the EKG such an e-function curve which is necessary for the exact compensation of the overshoot. In addition, unusually high hum amplitudes can be falsely evaluated as interference pulse peaks by the interference pulse peak-recognition installation, so that this can lead to unnecessary signal suppressions and possibly even signal suppressions which falsify the test result.